1. Field of the Invention
This invention relates to material handling systems and in particular to an automated system for the movement of work pieces within a manufacturing facility.
2. Description of the Relevant Art
Manufacturing facilities often employ material handling systems to move materials in various states of production (i.e., work pieces) between processing locations. Production states of work pieces vary from raw materials to finished products. Containers or carriers are commonly used to move the work pieces from one processing location to another during the manufacturing process. Typically, a transportation system is employed (e.g., a conveyor belt system) for moving the containers from one processing location to another within the system. Work pieces are typically placed into containers, and the work-piece-containing containers are transported between processing locations. Needed empty containers are generated as work pieces are removed from containers in order to be processed.
An example manufacturing facility is one which fabricates integrated circuits on semiconductor wafers. Such a facility typically includes a plurality of wafer fabrication process tools which perform various fabrication process steps upon groupings of semiconductor wafers called"lots". Each process tool typically has an accompanying stock area for storing containers of wafers waiting to be processed by the tool as well as wafers having already been processed by the tool. A common example of such a container is a wafer cassette or"boat" adapted for holding one or more semiconductor wafers.
Chemical mechanical polish (CMP) techniques are increasingly employed in the fabrication of integrated circuits. CMP techniques are used to planarize exposed upper surfaces of dielectric layers formed between layers of electrical conductors (i.e., interconnects). CMP combines chemical etching and mechanical buffing to remove raised features on the exposed upper surfaces. In a typical CMP process, a semiconductor wafer is mounted on a rotating holder and lowered onto a rotating surface flooded with a mild etchant solution, generally defined as a silica slurry. The etchant grows a thin layer on the exposed wafer surface that is almost simultaneously removed by the buffing action. The net effect is a very controlled polishing process capable of incredible flatness.
One problem with CMP techniques is that they produce large amounts of contaminants, including particulates, metallic ions, and chemical substances. The destructive effects of those contaminants is readily apparent in the overall performance of VLSI or ULSI devices. Any contaminants attributed to the slurry, chemical reactant, or buff/etch byproduct, which is thereafter introduced into other fabrication operations, severely compromises the success of those operations. For example, ingress of contaminants from the CMP operation to the thermal furnaces used for growing oxide, or to the chambers used for implanting ions, would negatively impact the resultant grown oxide or junction profile.
Without adequately preventing deposition of CMP-derived contaminants on semiconductor wafers undergoing other fabrication operations, CMP cannot be successfully employed. One way to minimize deposition of CMP-derived contaminants on semiconductor wafers undergoing other (i.e., non-CMP) fabrication operations would be to perform the CMP process in an area isolated (i.e., hermetically sealed) from other fabrication areas. Maintaining separate the CMP area from the other fabrication areas begins by installing a wall between those areas. Wafers must, however, be transported between the respective areas so that CMP can be incorporated within the process flow.
Transport of wafers between CMP and non-CMP areas entails passing the wafers through a door separating the areas. The door, depending upon sophistication, can be a load lock chamber adapted for passing a wafer-containing container. The wafer or wafers are transported in the container through the chamber from one area to another area. The load lock comprises an air circulation and filtration system which effectively flushes the ambient air surrounding the wafers. Unfortunately, however, the load lock by itself cannot in most instances remove contaminants from the surface of wafer-containing containers. The containers pick up contaminants while in the CMP fabrication area. When the containers passes through the load lock unit, those contaminants are not always flushed from the containers in the load lock. As a result, containers passed from one area to another may have contaminants clinging to them which may come loose and find their way onto the wafers.
It is therefore desirable to minimize the opportunity for a contaminated container to pass to and from a CMP area. An effective method of preventing passage of container entrained contaminants into what should be a "clean room" environment from a relatively dirty CMP room is to pass only the wafers through the wall and not the containers in which they reside. Such wafer transfer systems require coordinated efforts on both sides of the wall.
Robotic arms are now available which are able to accomplish many tedious and repetitive tasks previously performed by humans. Unlike a human, however, a robotic arm tirelessly performs such a task the same way every time, reducing variability in both the end result and the amount of time required to accomplish the task. The use of one or more robotic arms in a manufacturing process thus adds an element of predictability to the process.
Typically, if an empty container is not available when needed by a process tool, an empty container must be transported from another location in the system. Transportation of the empty container requires time, causing a delay in the processing of a wafer lot. Such time delays result in inefficient use of the process tool. The cumulative cost of such time delays may be substantial in a large semiconductor fabrication facility having several process tools which are costly to purchase, operate and maintain.
An obvious solution to the problem is to provide a relatively large number of empty containers in order to minimize the delay times. This solution may be prohibitive, however, both in terms of initial container costs and container storage costs. An increased number of empty containers requires more and/or larger stock areas for storage, and does not necessarily reduce the number of required container moves or increase production efficiency. Adequate distribution of the containers must be accomplished such that a sufficient number of empty containers are available when and where they are needed.
It is therefore desirable to have an automated material handling system for a manufacturing facility divided into separate fabrication areas. Such an automated material handling system would plan and carry out automated wafer transfer operations designed to pass only work pieces (e.g., semiconductor wafers) through walls separating fabrication areas, and not the containers used to hold the work pieces. Automating the wafer transfer operations would reduce the variability characteristic of manual operations. The desired automated material handling system would also manage the distribution of empty containers within the manufacturing system. Adequate distribution of empty containers would reduce the total number of empty containers required, reduce the number of required container moves within the system, reduce the required number and/or sizes of stock areas, and increase the overall production efficiency of the manufacturing system.